Processing and transmission of information with light requires creation of integrated optical circuits. While the idea is not novel, integrated circuits with the use of light do not repeat the success of electronic integrated circuits, while most important active and non-linear optic elements like lasers, amplifiers, detectors, and fast saturating absorbers, are routinely made in planar waveguides with microlithography, then diced and connected with optical fibers. It is much like the use of transistors before the invention of electronic integrated circuits. One of the main reasons is the problem of interconnection. Electric current easily follows through bends of a conductor, thereby facilitating interconnections among several layers. The light tends to propagate in a straight line; therefore, interconnections among several layers are difficult. Sometimes active elements are interconnected by ridge waveguides in a single waveguide, but this method is limited due to the crossing of ridge waveguides in a single layer. Thus, there is a great need for interconnecting many optical elements in a single waveguide.
Attempts have been made heretofore to provide planar optical devices by interconnecting many optical devices on a single substrate. For example, U.S. Patent Application Publication No. 20070034730 published in 2007 (inventor T. Mossberg, et al.) discloses a multimode planar waveguide spectral filter that comprises a planar optical waveguide having at least one set of diffractive elements. The waveguide confines in one transverse dimension an optical signal propagating in two other dimensions therein. The waveguide supports multiple transverse modes. Each diffractive element set routes a diffracted portion of the optical signal between input and output ports, the optical signal being one that propagates in the planar waveguide and is diffracted by diffractive elements. The diffracted portion of the optical signal reaches the output port as a superposition of multiple transverse modes. A multimode optical source may launch the optical signal into the planar waveguide through the corresponding input optical port as a superposition of multiple transverse modes. A multimode output waveguide may receive the diffracted portion of the optical signal through the output port. Multiple diffractive element sets may route corresponding diffracted portions of an optical signal between one or more corresponding input and output ports. The device involves the principle of refractive index modulation.
U.S. Patent Application Publication No. 20060233493 published in 2006 (inventor T. Mossberg, et al.) discloses a holographic spectral filter. According to one embodiment, the device of the invention comprises a planar waveguide appropriate to contain a programmed planar holographic spectral filtering device. Input and output signals propagate within the planar holographic substrate in the x-y plane. The planar holographic substrate, or slab, is typically constructed of a material sufficiently transparent at the intended operational wavelength of the device so that unacceptable loss does not accrue from absorption as signals propagate through the programmed holographic device. Typical substrate materials include silica (SiO2), which is transmissive over much of the visible and near-infrared spectral region, polymers, and silicon. The thickness of the planar substrate is preferably set to a value low enough to ensure that only a relatively low number of transverse (z) modes is allowed, or more specifically, that the allowed transverse (z) modes do not experience significant modal dispersion when passing through the programmed holographic device.
U.S. Patent Application Publication No. 20070053635 published in 2007 (inventor D. Iazikov, et al) discloses transmission grating designed by computed interference between simulated optical signals and fabricated by reduction lithography. More specifically, the method comprises computing an interference pattern between a simulated design input optical signal and a simulated design output optical signal and computationally deriving an arrangement of at least one diffractive element set from the computed interference pattern. The interference pattern is computed in a transmission grating region, with the input and output optical signals each propagating through the transmission grating region as substantially unconfined optical beams. The arrangement of the diffractive element set is computationally derived so that when the diffractive element set, thus arranged, is formed in or on a transmission grating, each diffractive element set routes a corresponding diffracted portion of an input optical signal between corresponding input and output optical ports, the signal being one that is incident on and transmitted by the transmission grating. This method can further comprise forming the set of diffractive elements in or on the transmission grating according to the derived arrangement.
U.S. Patent Application Publication No. 20060126992 published in 2006 (inventor T. Hashimoto, et al.) discloses a wave transmission medium that includes an input port and an output port. The first and the second field distributions are obtained by numerical calculations. The first field distribution distributes the forward propagation light launched into the input port. The second field distribution distributes the reverse propagation light resulting from reversely transmitting from the output port side an output field that is sent from the output port when an optical signal is launched into the input port. A spatial refractive index distribution is calculated on the basis of both field distributions such that the phase difference between the propagation light and reverse propagation light is eliminated at individual points (x, z) in the medium. The elements of this system are also mounted on a planar substrate.
U.S. Patent Application Publication No. 20040036933 published in 2004 (inventor V. Yankov, et al.) discloses a planar holographic multiplexer/demultiplexer that is characterized by low manufacturing cost, reduced signal distortion, high wavelength selectivity, high light efficiency, reduced cross-talk, and easy integration with other planar devices at a lower manufacturing cost. The planar waveguide of the device includes a holographic element that separates and combines predetermined (preselected) light wavelengths. The holographic element includes a plurality of holograms that reflect predetermined light wavelengths from an incoming optical beam to a plurality of different focal points, each predetermined wavelength representing the center wavelength of a distinct channel. Advantageously, a plurality of superposed holograms may be formed by a plurality of structures, each hologram reflecting a distinct center wavelength to represent a distinct channel to provide discrete dispersion. When used as a demultiplexer, the holographic element spatially separates light of different wavelengths and when reversing the direction of light propagation, the holographic element may be used as a multiplexer to focus several optical beams having different wavelengths into a single beam containing all of the different wavelengths.
However, in all aforementioned prior-art devices, for transformation of an input beam into an output beam, the inventors use holographic gratings with known functional properties determined by their parameters and geometry. Therefore, positions and optical parameters of the input and output beams strictly depend on the geometry of the grating, and this significantly limits design of the optical structure. Another disadvantage of the known planar holographic devices is that they have a limited number of light-transmitting channels since each holographic pattern element works only with one or two channels.